In order to determine the molecular mechanism of genetic control at the promoter and attenuator of the histidine operon of Salmonella typhimurium, the DNA sequence of the entire genetic control region will be determined, beginning as far as necessary (perhaps 1200 b.p.) before the first structural gene (hisG) of the operon. Advantage will be taken of the existence of some 150 known and studied mutations (making this the most mutated genetic control region known). The exact nucleotide changes of many of these mutations will be determined, in order to clarify, support or alter the mechanistic interpretation of the wild-type sequence which can be made from features of the sequence and previous knowledge of the physiological and genetic behaviour of this genetic control system. The DNA sequencing technique of choice is the chain-termination method of Sanger et al., using as template the DNA of a single-stranded M13-histidine transducing phage constructed for the purpose using in vitro genetic engineering. As an adjunct under this project, the entire sequence of the first two structural genes of the operon (hisG and hisD) will also be determined. This sequence includes the sites of most of the mutations that are reverted by the Salmonella/microsome carcinogen test of Ames, in addition to many other mutational sites known to bacterial geneticists.